Microwave and millimeter-wave communications systems are used in many applications, including satellite communications, terrestrial point-to-point communications, and backhaul communications for cellular networks. Typically, a communications transmitter includes a high power amplifier to increase the power of the signal to levels adequate to reach a distant receiver with sufficient strength. It is important that these communications transmitters preserve the fidelity or the “linearity” of the communications signals to avoid unnecessary distortion.
Typically, a high power amplifier will add some undesirable distortion to signals during the amplification process. For example, as the power for an input drive signal increases, an amplifier will amplify the drive signal by a proportionate gain. However, when the power of the input drive signal reaches a certain level, the amplifier begins to become saturated and is no longer capable of amplifying the drive signal by a proportionate gain. In other words, as the amplifier becomes saturated at these higher input power levels of the drive signal, the amplifier begins to add saturation distortion to the amplified output. Thus, the high-power amplifier will not produce sufficient gain and adds amplitude distortion to the output signal above a certain level of input drive power. In addition, the phase of the output signal can also become distorted as the amplifier saturates which further compounds the saturation distortion problem. This amplitude (also referred to as magnitude) and phase distortion result in a loss of fidelity of the output signal, and will limit the capacity of the communications system.
Conventional amplifier designers have attempted to mitigate this distortion characteristic of the amplifier by coupling a predistorter device to the amplifier. The predistorter attempts to counteract the distortion characteristics of the amplifier by preconditioning the drive signal before amplification. For example, a predistorter may add an inverse gain magnitude and inverse gain phase saturation distortion characteristics to the drive signal before amplification. Thus, when the predistorted drive signal is amplified, the inverse magnitude and inverse phase saturation distortion characteristics will counteract the saturation magnitude and phase distortion added during amplification. As a result, the operating power levels of an amplifier may be extended into much higher power levels without exhibiting much distortion. These devices are said to extend the linear output power of the high power amplifier. The benefits of using linearization techniques to extend the useful power of a high-power amplifier are well documented.
However, many amplification applications require the use of more than one frequency, and some applications may even require the use of an entire broad frequency band. Generally, power amplifiers exhibit different saturation distortion characteristics for drive signals of different frequencies. In other words, the amplifier distortion characteristics of a particular amplifier will change over the frequency of a band of interest. This difference in saturation distortion characteristics of an amplifier becomes even more pronounced for wideband applications that require a greater range in frequencies in an operating bandwidth. It is often quite difficult to design a conventional predistorter to counteract the amplifier's gain magnitude and phase saturation over a wide frequency band, especially if the amplifier's gain magnitude and phase saturation vary significantly over this band. As a result, while conventional predistorters may help extend the operating range for an amplifier at one particular frequency, those conventional predistorters are often inadequate for applications that require amplification for an entire band of frequencies.